96 Tacoma rough shift

[Brett!]

I bought a Tacoma with an auto trans from a guy for $1300, put some work into it and have been letting my parents use it. Aside from a small ps the only problem I am having is a very hard jolt every time you start from a stop (1st to 2nd gear?). I know the trans is a used one and not the original. Truck has 335k miles. Drives good other then the rough shift. Trans fluid looks clean and there is plenty in there. I noticed the mount under the trans is falling apart and loose so I plan to change that... But I am not expecting that to fix the problem. No CEL so I am going to take to a trans place and get a diagnostic. Thought id ask some of the experts here.

Thanks!